When adding a month to the date, all lesser components (days, hours, ...)
won't be changed unless the resulting date would be invalid: for example,
-Jan 31 + 1 month will be Feb 28, not (non existing) Feb 31.
+Jan 31 + 1 month will be Feb 28, not (non-existing) Feb 31.
Because of this feature, adding and subtracting back again the same
wxDateSpan will {\bf not}, in general give back the original date: Feb 28 - 1
<wx/datetime.h>
+\wxheading{Library}
+
+\helpref{wxBase}{librarieslist}
+
\wxheading{See also}
\helpref{Date classes overview}{wxdatetimeoverview},\rtfsp